I wouldn't worry about saying "umm" too much. When I'm interviewing someone, I'm much more interested in content than delivery. If you drop an "umm," then try to catch yourself doing it, you will only manage to lose the message you're trying to convey. Obama said "umm" a lot and still got one of the most powerful jobs in the world. Any research and understanding of the company and what they do is a huge plus. If it sounds like a candidate hasn't even looked at the front page of our website, it's difficult to take them seriously for the rest of the interview. This one is more my personal preference, but if you're asked what your biggest weakness is, don't say you're "too much of a perfectionist" or something lame like that. I would rather someone give me an honest answer, since it shows me that they can recognize their own weaknesses and understand that they are, in fact, weaknesses. It also makes me more likely to believe them when they tell me about their strengths. As for the pre-interview jitters, working out is an excellent idea. Not only can it help relax you, but it can also make you feel stronger and more confident, as if you've already put in a day's hard labor.
